A $1.5 billion expansion of Minneapolis-St. Paul International Airport is gaining momentum amid uncertainty about future flying needs and growing scrutiny of the project's impact on surrounding neighborhoods.

The proposed expansion would increase airplane noise for thousands of metro homes. Most of them received sound-proofing from the airport and won't get additional help. But 1,131 in Minneapolis are expected to get subsidized sound-proofing.

The agency that runs the airport said the expansion is needed to relieve current congestion and meet future demand. The Metropolitan Airports Commission recently released a report on the environmental impact of the expansion. That will be the focus of public sessions in the next few weeks that could help shape a final decision.

But even some members of the airports commission question whether demand will justify the expansion.

"I'm a little concerned about where the economy is going to be in the next decade or two," said Commissioner Mike Landy. "I've seen some airports ... Cincinnati is one, that have been entirely turned on their head. I just want to be a little careful."

"I don't think it's a done deal," said Commissioner Greg Foster, who represents Minneapolis. "There's skepticism from the commissioners as to how much we really need to proceed with. We need to be very cautious about spending a lot of money without the clear path to increased traffic."

The plan calls for remodeling concourses and building a new international wing and a parking ramp and paying for it with airport revenues, mostly from passengers and airlines. The expansion would benefit the airport's dominant carrier, Delta Air Lines, by consolidating its operations into one terminal and moving other major airlines to a less popular one with low-cost carriers.

The airport is recommending the move over a less expensive expansion that would leave United and other major airlines with Delta and its partners at the Lindbergh terminal and keep low-cost carriers at the Humphrey terminal.

The plan is based on assumptions that Delta will maintain a major hub at the airport, larger aircraft will carry more passengers and the Twin Cities business community will resume expanding after economic hard times.

"Unless the economy totally collapses, there is going to be growth in the market here," said Dennis Probst, executive vice president of the airports commission. Citing the presence of numerous Fortune 500 firms in the region, he said, "The business community is going to continue to travel unless folks start pulling out of here."

Still, Probst acknowledges that estimating future demand can be dicey.

"These are forecasts," he said. "I can guarantee ... if you pick a date out there and pull the number off our chart, it's going to be wrong."

He said any expansion would occur in stages and be phased in according to demand.

Arrivals and departures are expected to increase from 437,075 in 2010 to 484,879 in 2020. By 2025, the airport predicts that number will reach 526,040.

That would fall short of the 541,093 flights at the airport in 2004, when air traffic peaked before economic problems hit the airlines and general economy.

But because airlines are replacing 50-seat planes with larger regional aircraft and flying them to capacity, the number of passengers is forecast to reach 50 million by 2030 compared with 37.6 million at the peak before the recession, Probst said.

Others skeptical

The prediction is "really, really aggressive," said Michael Boyd, an industry analyst in Denver who forecasts trends for airports and airlines. "They have to base it on what Delta Air Lines may or may not do. That's hard."

Cincinnati saw a major drop in traffic after Delta slashed its hub operations there. "Pittsburgh built a brand-new terminal," Boyd said. "They said if US Airways pulled the plug, somebody else will be here. Now they have a lot of plywood-boarded-up buildings."

The expansion isn't needed, says Minneapolis City Council member John Quincy, who has followed airport issues closely over many years.

"We've built an awfully big box already," Quincy said. Citing high grades for the airport from critics and uncertainty about demand, he asked, "Why do you need to spend another billion dollars on an airport expansion?"

The report on the environmental impact concludes that it wouldn't be significant and that roughly as many homes would face increased noise even without the expansion, assuming the same increase in demand for flights.

It will be rolled out for discussion at meetings this week in Minneapolis and in Eagan, which along with some areas of Richfield, Bloomington and Mendota Heights, would experience rising levels of noise. The report forecasts 2,703 metro-area homes would experience more noise by 2020 than in 2010, but most received sound-proofing when noise reached similar levels during peak travel years ago and won't get any more. Only 1,131 homes in Minneapolis would get noise abatement for the first time or additional abatement. They would be eligible for air-conditioning, reimbursed upgrades and other improvements.

The city of Minneapolis last week prepared a statement critical of the findings.

"There are serious problems with how the MAC is developing the noise exposure maps, which are the basis for understanding how airplane noise affects the community, as well as determining which homes receive future noise mitigation," a draft read. "It is likely there will be major environmental impacts."